It's impossible to imagine Gossip Girl without Blake Lively or Arrow without Stephen Amell, and that's all because of David Rapaport, who has served as the casting director on some of The CW's biggest shows since 2008.

"I love seeing movies and TV shows and thinking, Who would I cast in that role? Or reimagining where people might fit," Rapaport told BuzzFeed News while sitting in one of his casting suites in Burbank, California. "I feel like you need to learn that gut instinct of what works tonally for [your show] — that's a learned instinct and it's also a learned thing to trust your instincts."

So far, those instincts haven't betrayed him. Along with his team, Rapaport has helped hire dozens of other actors who've gone on to help define The CW network. "There is no easy decision because there are maybe 50 people who make that final decision — studio, network, producers — so even though I think I have the right actor for the role, it is always a process to get them cast," he said. "I kind of have to fall in love with all these people I cast because I need to assume the world is also going to fall in love with them, so I have to see that ability in them and I fell in love with each and every one of these actors for what they brought to the character."

Here, in his own, lightly edited words, Rapaport details the highs, lows, hurdles, and discoveries he's made over nearly a decade of working with The CW.

Blake Lively (Serena van der Woodsen, Gossip Girl)

The CW

"I met with Josh Schwartz and Stephanie Savage — and this was at the height of The O.C. — and they basically said, 'We need the next Mischa Barton and Adam Brody,'" Rapaport recalled of his first meeting with the executive producers. "At the time, they were the biggest names in television. I said, 'Absolutely. Count on us to find them for you.' I don't know where the fuck I thought I was going to find them — I don't even know where I got the ego to say that I could do that, but it was the challenge placed on the table, but I knew that it was something we could do.

"Blake Lively was literally the only person I could think of for Serena. I had seen her in Accepted [a 2006 comedy co-starring Justin Long] and at that time, there were no Lena Dunhams or Zooey Deschanels — it was all about beauty and glamour, but being approachable and Blake was the ultimate It Girl. I was terrified we wouldn't get her or they wouldn't like her because, honestly, they show lived and died on her and I had absolutely no other ideas. And I read so many people. I think at the end of the day, we tested Blake, Katie Cassidy, who is now on Arrow, and this girl who was [Serena] on the Gossip Girl books. Not even her face, just her legs. But it made sense.

"Blake was the front-runner. We did a screen test with her and the note we got back was that she was 'sunny California' and they said, 'I don't think we'll buy her as an Upper East Side debutante.' And Josh said, 'Well, Mischa Barton is from New York and I sold her as a sunny California girl; there's no reason people won't.' So we did another screen test with Blake and all we did was straighten her hair to make her look a little more sophisticated, so to speak. What's ironic is, in the pilot, she ends up having the wavy hair. From there on out, so much of the casting on Gossip Girl became about the hair because Leighton Meester is naturally blonde and the way we got her the job was we dyed her hair. Ultimately they were the best actors for it and brought something really special to their roles."

Leighton Meester (Blair Waldorf, Gossip Girl)

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"Leighton is a little bit colder than Blake but I like that," Rapaport said of casting the two as Gossip Girl's best frenemies. "A lot of actors come in and want to please me and are super-kind [and] trying to be my best friend and they flirt and the whole thing, and that's great, but I look for people to come in and be professional and to be kind and courteous. I'm not looking for you to love me; I'm looking for your personality. I'm looking for your uniqueness, so to speak. And Leighton wasn't a bitch when she came in, but she wasn't overly nice or overly generous. She was professional and she did the job and played the role well, and what I liked about her is that she owned who she was. She wasn't going to kiss everyone's ass and that was very Blair.

"She was obviously playing a character, but I wouldn't have cast her as Serena because she's not open like that and that's what makes her interesting and that's why I feel like you go back and think, There is nobody else that could have played these characters. I think the network initially pitched us Ashley Olsen and Rumer Willis for Blair and Serena — I love Rumer, she's grown into quite an amazing actress and person, but that, to me, was based on protecting the investment of a television show where creatively, these lesser-known girls really captured the essence of the show and carried it for six years."

Ed Westwick (Chuck Bass, Gossip Girl)

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"We read him initially for Nate," Rapaport said. "He read on tape in London and flew out to read for Chuck. He's menacing and scary. but there's a twinkle in his eye. You want to hate him, but you would also probably sleep with him. He's one of those guys you hate for always getting away with things, but you also want to hang out with him and see what he's up to next. He's the guy that's going to give you a joint for the first time or get you drunk for the first time, so you know he's wrong for you, but he's fun.

"Ed's visa did not come through until, I think, the day they shot the pilot because he's from the U.K., so we had to cast backups. I don't even know how many guys I read and the producers — I don't think they'd be mad at me for saying this — would not watch any of the tapes. They said, 'It has to be Ed.' And the studio and network told me it may not be Ed because we may not get his visa in time. I was in this standstill where I didn't know what the fuck to do because we needed to recast it, but they wouldn't look at the tapes. Luckily they were so strong in their belief for Ed.

"It's funny because on a network like The CW, where beauty does come first a lot, and you're selling the fantasy of this world, it was a lot about the looks, so we had to balance that out with the guys and Ed Westwick wasn't as traditionally handsome as Chace Crawford, but I found him much sexier and darker and edgier."

Taylor Momsen (Jenny Humphrey, Gossip Girl)

The CW

"I certainly did not see that coming ... I don't think anybody did," Rapaport said, with a laugh, of Jenny's trajectory, going from innocent school girl to raccoon-eyed schemer during the show's run. "Taylor actually read on tape and we cast her off tape, so we never had her here in person. We were looking for a girl who was on the brink of becoming a woman — I don't think we saw that goth side yet [because] she was still really young; she was maybe, like, 13 when we cast her. Again, I don't think anyone had met her in person and everything was fine as far as I knew and then it kind of took a turn.

"When people audition, like in any sort of job interview, they show you their best selves. You don't really get a sense of them. I kind of see myself a bit as an HR person so I try and get a sense of are they going to be good on set? Are they going to be professional? On time? Courteous? In the two or three auditions you see these people on, they all seem great. Some of them lose their shit and you figure out very quickly it's not a reliable person. But she's great. [One show] shouldn't define you. I support all of these actors doing things outside the box," he added of her music career.

Katie Cassidy (Juliet Sharp, Gossip Girl)

The CW

"She is so reliable, she's so beautiful, she's so talented, she's kind of everything that you want in an actor on a CW show," he said of Cassidy, who he later cast on Arrow. "I read a ton of actors and my process is I pre-read a bunch of actors to narrow down the field to bring to producers, but there's a small group of actors I will bring in project after project; I know them, I trust them, I know they're going to perform, and it's just a matter of finding them the right role. Katie is one of those people. She always brings something really interesting to the table; she's fun to watch. She's kind of electric. You can't quite put your finger on it, but when she's on screen, something magical happens and you can't take your eyes off of her. And my dad really likes her."

Jessica Lowndes (Adrianna Tate-Duncan, 90210)

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"I think I thought she was going to die initially," he said of Lowndes' character. "I thought this was going to be a one-season thing. I didn't think she was going to last as long as she did, but the producers loved her and she did such a great job. Her manager produced a series I did called Masters of Horror and Fear Itself and I read her for 23 episodes of that show to try and get her on it and never could for some reason. When this came along, she was just perfect. She was sexy and kind of bitchy and fun and campy."

Kellan Lutz (George Evans, 90210)

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"I thought he was going to become a series regular, but I got a call from his manager saying, 'Kellan is going to pull out of the show because he's going to be doing this movie about teenage vampires based on this big book and it's going to be huge,'" Rapaport remembered. "I remember thinking, You're such a fucking idiot. He's going to pull out of the biggest show ever to do a vampire teen movie? That was a situation where I was wrong," he added with a laugh.

Matt Lanter (Liam Court, 90210)

The CW

"We were looking for a new love interest and Finn Wittrock [of American Horror Story: Freak Show] was in the running for it," Rapaport said. "He read for Liam and tested. I think AnnaLynne [McCord] liked him, but Matt won. I didn't really realize that role was going to be as big as it was."

Rumer Willis (Gia Mannetti, 90210)

The CW

"I'm not a huge fan of stunt casting for ratings — I understand why we do that and I understand the business versus the creative. But for me, I'm always more interested in discovering someone," Rapaport said. "I think it was important for 90210 to place itself in Beverly Hills — we had the Kardashians on, we had Prince Michael on; so in that sense, I get it. But with Rumer, she auditioned and was genuinely the best person for the role. She was an exciting get and got us a lot of press, but she also told a really interesting story between her and Jessica Lowndes [about] being lesbians. I was glad someone at her level was brave enough at that time to tell a story that wasn't on every other show. That casting made a lot of sense to me."

Trai Byers (Alec Martin, 90210)

The CW

"I didn't birth these people, but I gave them opportunities and I saw something in them and the timing was right and other people saw something too," said Rapaport. "I look at someone like Trai Byers, who is on Empire right now, and right out of Yale, he came in and auditioned for 90210 and I thought, We are so lucky to get this guy. He should be doing better things. He's so talented and has such star potential.

"He also came in to audition for The Tomorrow People — he came in for the John role — and he bombed. He gave one of the worst auditions I have ever seen. I pulled him out and said, 'Dude, I believe in you so much and you have such a potentially massive career ahead of you, but you can't fuck up like this. You have to get your head in the game.' I became this coach out of nowhere, but I feel like I have opportunities where I can guide people and he was somebody I felt strongly about and I look at him now! He's on the biggest show on TV and it's amazing. I don't want to take responsibility, but I feel like an important part of these people's trajectories."

Stephen Amell (Oliver Queen, Arrow)

The CW

"I had just cast him on 90210 where he played this war hero who had come home from fighting and we were looking for the ultimate superhero on Arrow and physically he looked the part," Rapaport said. "We wanted someone with a darkness and an edge to him, but someone who looked like He-Man. I brought his picture with me to the initial meeting for Arrow and everyone responded to the picture and I thought, I should just bring him in first because he's it — and he was it. That made my life a lot easier. We didn't test that many people. It was him and nobody else.

"The Vampire Diaries was on the air already, so we were sold as a three-hander between Stephen, Katie Cassidy, and Colin Donnell — a tripod if you will. That's how we cast it and then the posters came out and Stephen was shirtless. First of all, we never saw him without his shirt on; I knew he was a physical guy, but I'd never seen him do stunts or anything like that and all of a sudden in the pilot, he's doing his own stunts. He became famous for being shirtless. The poster was him shirtless and none of the other actors were on the poster, and as talented and amazing as they were, it became clear the show was about 'The Arrow,' so that really changed the concept for us because initially I saw it as more of an ensemble and cast it that way."

Emily Bett Rickards (Felicity Smoak, Arrow)

Cate Cameron / The CW

"Ultimately, my goal is to cast the best people for the show, with that potential pop factor, and Emily is the perfect example," he said of the actor who went from guest star in Season 1 to series lead in Season 2. "That could have just been a one-line role but she popped. We've also cast roles that were supposed to be 9-, 10-episode arcs that have turned into one episode and they're killed off. So, it goes both ways. I never know. That's a difficult part of the process."

David Ramsey (John Diggle, Arrow)

Jack Rowand / The CW

"If I remember correctly, Diggle was the last role to be cast and it was such a small role in the pilot," he said. "We did a studio and network test for it with maybe one page of dialogue. David is such a phenomenal actor. I'd known his work for a while and we kind of had a last-minute session. It wasn't a forgettable character, but it was a last minute addition to finishing this pilot and we brought in five guys — he was the best. We tested him the same day we auditioned him and we just got really lucky because I don't think anyone was expecting the role to be that big and we weren't expecting that much from him, so we all felt so lucky to have such a great actor doing such a supporting part. Again, like Emily's character, Diggle really grew throughout the season and now he's such an important part of the team."

Willa Holland (Thea Queen, Arrow)

Jack Rowand / The CW

"Melissa Benoist read for Thea," Rapaport said of the actor he later cast as Supergirl. "We read a bunch of people for Thea and it kept coming back to Willa because, with all these characters on Arrow, there was a duality about who they were on the surface and the darkness behind that. Even being so young, Willa's lived a bit and that's what makes her so interesting.

"I had a harder time convincing her to consider the Gossip Girl spin-off, which she ultimately passed on, then I did getting her in for Arrow. I think at that point, enough time had passed [after she co-starred on The O.C.] and it was an exciting, different role with the opportunity to potentially become a comic book hero in the end. I think everyone saw how different and dark and gritty this project was going to be."

Caity Lotz (Sara Lance, Arrow)

Cate Cameron / The CW

"That was a tough one because we were recasting the role of Sara Lance from the pilot," he said. (Jacqueline MacInnes Wood originated the part.) "I had seen Caity on Mad Men a few years earlier and brought her in for 90210. She didn't get that part. Then I brought her in for this and she didn't get Sara right away, so we kept looking. But it was one of those things where I felt like, We have to go back to our initial instincts because she was so great. And now we've ended up building DC's Legends of Tomorrow around her."

Robbie Amell (Stephen Jameson, The Tomorrow People)

The CW

"Robbie read to play Arrow, initially, and was a little bit too young," Rapaport said. "Robbie had read for Gossip Girl and 90210 as well. We wanted him for John, the older guy that Luke Mitchell ended up getting cast as, and I think it was Peter Roth [the chief executive of Warner Brothers Television] who kind of turned it on its head and said we should cast Robbie as Stephen and Luke as John. All of sudden, it just made sense. I think Robbie was attached to [NBC's 1600 Penn], so he wasn't even technically available and we were all really nervous, but it seemed like a safe, second position because that was likely going to get canceled — and it did. I think we made the right choice because Robbie is such a star."

Grant Gustin (Barry Allen, The Flash)

Jack Rowand/The CW

"I saw Grant Gustin on Glee and thought, Wow, this kid is a star. There's something really, really special about him. Then I cast him in a very similar villainous role on 90210, but every time I brought him in, he was this nice, goofy kid and he just wasn't playing these characters based on who he was. So when The Flash came up and we wanted this relatable, goofy guy who became a superhero, he just made sense to me.

"But we had to see a lot of people. We saw Andy Mientus, who we ended up casting as The Pied Piper [on The Flash], and Jeremy Jordan, who we ended up casting on Supergirl. I sent an email to Greg [Berlanti, executive producer of Arrow, The Flash, The Tomorrow People, Supergirl, and DC's Legends of Tomorrow] about a year ago with all the people on our shows who had read for other roles and Grant Gustin had read for Thea's boyfriend in Season 1 of Arrow, and I thought, How screwed would I have been if we cast him then?!?"

Candice Patton (Iris West, The Flash)

The CW

"That was a really hard role to cast," Rapaport said. "We had seen Candice initially, but I think we were too early on in the process to cast her so we ended up testing Keke Palmer and this other girl, but they felt a little too young next to Grant. When building an ensemble, you have to do those chemistry reads to find out who works best together. We went back and looked at the tapes and Candice shines — when she smiles, it's out of control — so we thought, Let's do another chemistry read and we'll bring Candice and one other girl in and it was just magic. Everyone in the room and looked at eachother and we all just knew it. It just made total sense."

Carlos Valdes (Cisco Ramon, The Flash)

The CW

"I saw a ton of people for Cisco. We cast him out of New York. He was in Once on Broadway, but I don't think he'd ever been on TV before. It was one of those things where we saw the tape, and sometimes it doesn't translate on tape, but he was so natural and funny and brought so much more than we ever even imagined to the character. We had some guys here that could have done it, but not in the way he's played the role. To me, Carlos was such a standout of the first season."

"I think the stakes are higher, so the producers have raised the bar a bit because they want a huge hit," he said of the 2016 series, which is spun off from both Arrow and The Flash. "It's easier to cast in the sense that more people are open to these shows because of their success. When you have these actors like Victor Garber already in, the producers are more open to discoveries. So we kept someone like Ciara Renee — who read for Iris West — in mind and she just knocked it out of the park. Franz [Drameh] is someone I had seen in Attack the Block a couple of years ago and brought him in to read for Jay Jackson, the Firestorm role — him and Keiynan Lonsdale, actually. And it was Greg who said, 'Cast Franz as Jay Jackson and Keiynan as Wally West on The Flash.' And I was like, 'Yes! Two birds with one stone!' It makes it easier because I think people trust me and trust that I know what I'm doing and Greg and the studio and the network trust my instincts when I fight for people."